Phil Hosegood

Assessing wave energy effects on biodiversity: the Wave Hub experience

Marine renewable energy installations harnessing energy from wind, wave and tidal resources are likely to become a large part of the future energy mix worldwide. The potential to gather energy from waves has recently seen increasing interest, with pilot developments in several nations. Although technology to harness wave energy lags behind that of wind and tidal generation, it has the potential to contribute significantly to energy production. As wave energy technology matures and becomes more widespread, it is likely to result in further transformation of our coastal seas. Such changes are accompanied by uncertainty regarding their impacts on biodiversity. To date, impacts have not been assessed, as wave energy converters have yet to be fully developed. Therefore, there is a pressing need to build a framework of understanding regarding the potential impacts of these technologies, underpinned by methodologies that are transferable and scalable across sites to facilitate formal meta-analysis. We first review the potential positive and negative effects of wave energy generation, and then, with specific reference to our work at the Wave Hub (a wave energy test site in southwest England, UK), we set out the methodological approaches needed to assess possible effects of wave energy on biodiversity. We highlight the need for national and international research clusters to accelerate the implementation of wave energy, within a coherent understanding of potential effects—both positive and negative.

Restratification of the Surface Mixed Layer with Submesoscale Lateral Density Gradients: Diagnosing the Importance of the Horizontal Dimension

Abstract A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of ∼100-m vertica...

Mixing within the interior of the Faeroe-Shetland Channel

Observations of the distribution of the density field, the dissipation rate of turbulent kinetic energy, e, the vertical diffusivity, Kz, and optical backscatter over a repeated cross-section of the FaeroeShetland Channel (FSC) are presented within the context of ocean mixing. Turbulence in the permanent pycnocline occurs in discrete vertical patches of thickness 20‐50 m coincident with layers of elevated vertical current shear magnitude, S 10 2 s 1 . The layers of elevated shear result from high-wavenumber internal waves which may break through shear instabilities as indicated by critical Richardson numbers, Ri 1. The internal waves are ubiquitous features of strong boundary currents flowing along continental shelves and are likely generated here by geostrophic adjustment of the pycnocline in response to lateral excursions of the boundary current over the Shetland slope or by the scattering of internal tide energy. The pycnocline as a whole exhibits Kz 10 4.5 m 2 s 1 , a factor of 3 larger than typical open ocean thermoclines but still implying a weak exchange of water mass properties across the density interface despite the confined and isolated patches of enhanced turbulence which elevate Kz by an order of magnitude. Both the boundary current and the deep interior are typified by weak stratification and locally high Kz 10 3 m 2 s 1 , but low turbulent buoyancy fluxes. The strongest observed turbulence and mixing in the channel, e O(10 7 Wk g 1 ) and Kz 10 3 m 2 s 1 respectively, is observed in the near-bed region over the Shetland slope and results from solibore propagation up the slope and the asymmetric response of the bottom boundary layer to the tidal currents. The overall contribution of the FSC to oceanic mixing would appear to be about half of the required canonical value of Kz 10 4 m 2 s 1 given the observed short duration and/or infrequent occurrence of the processes generating turbulence in the near-bed region, the spatially confined sporadic mixing patches in the pycnocline, and the low turbulent buoyancy fluxes in the interior and slope current.

Frontal circulation and submesoscale variability during the formation of a Southern Ocean mesoscale eddy

Observations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.

Seabird diving behaviour reveals the functional significance of shelf-sea fronts as foraging hotspots

Oceanic fronts are key habitats for a diverse range of marine predators, yet how they influence fine-scale foraging behaviour is poorly understood. Here, we investigated the dive behaviour of northern gannets Morus bassanus in relation to shelf-sea fronts. We GPS (global positioning system) tracked 53 breeding birds and examined the relationship between 1901 foraging dives (from time-depth recorders) and thermal fronts (identified via Earth Observation composite front mapping) in the Celtic Sea, Northeast Atlantic. We (i) used a habitat-use availability analysis to determine whether gannets preferentially dived at fronts, and (ii) compared dive characteristics in relation to fronts to investigate the functional significance of these oceanographic features. We found that relationships between gannet dive probabilities and fronts varied by frontal metric and sex. While both sexes were more likely to dive in the presence of seasonally persistent fronts, links to more ephemeral features were less clear. Here, males were positively correlated with distance to front and cross-front gradient strength, with the reverse for females. Both sexes performed two dive strategies: shallow V-shaped plunge dives with little or no active swim phase (92% of dives) and deeper U-shaped dives with an active pursuit phase of at least 3 s (8% of dives). When foraging around fronts, gannets were half as likely to engage in U-shaped dives compared with V-shaped dives, independent of sex. Moreover, V-shaped dive durations were significantly shortened around fronts. These behavioural responses support the assertion that fronts are important foraging habitats for marine predators, and suggest a possible mechanistic link between the two in terms of dive behaviour. This research also emphasizes the importance of cross-disciplinary research when attempting to understand marine ecosystems.

Mesoscale and Submesoscale Effects on Mixed Layer Depth in the Southern Ocean

The authors gratefully acknowledge support from the Natural Environment Research Council Awards NE/J010472/1 and NE/J009857/1.

Submesoscale Rossby waves on the Antarctic circumpolar current

Submesoscale Rossby waves are found along the Antarctic circumpolar current where they drive strong vertical circulations. The eastward-flowing Antarctic circumpolar current (ACC) plays a central role in the global ocean overturning circulation and facilitates the exchange of water between the ocean surface and interior. Submesoscale eddies and fronts with scales between 1 and 10 km are regularly observed in the upper ocean and are associated with strong vertical circulations and enhanced stratification. Despite their importance in other locations, comparatively little is known about submesoscales in the Southern Ocean. We present results from new observations, models, and theories showing that submesoscales are qualitatively changed by the strong jet associated with the ACC in the Scotia Sea, east of Drake Passage. Growing submesoscale disturbances develop along a dense filament and are transformed into submesoscale Rossby waves, which propagate upstream relative to the eastward jet. Unlike their counterparts in slower currents, the submesoscale Rossby waves do not destroy the underlying frontal structure. The development of submesoscale instabilities leads to strong net subduction of water associated with a dense outcropping filament, and later, the submesoscale Rossby waves are associated with intense vertical circulations.

A downslope propagating thermal front over the continental slope

In the ocean, internal frontal bores above sloping topography have many appearances,depending on the local density stratification, and on the angle and source of generation of the carrier wave.However, their common characteristics are a backward breaking wave, strong sediment resuspension, andrelatively cool (denser) water moving more or less upslope underneath warm (less dense) water. In thispaper, we present a rare example of a downslope moving front of cold water moving over near-bottomwarm water. Large backscatter is observed in the downslope moving front’s trailing edge, rather than theleading edge as is common in upslope moving fronts. Time series observations have been made during afortnight in summer, using a 101 m long array of high-resolution temperature sensors moored with anacoustic Doppler current profiler at 396 m depth in near-homogeneous waters, near a small canyon in thecontinental slope off the Malin shelf (West-Scotland, UK). Occurring between fronts that propagate upslopewith tidal periodicity, the rare downslope propagating one resembles a gravity current and includes strongconvective turbulence coming from the interior rather than the more usual frictionally generated turbulencearising from interaction with the seabed. Its turbulence is 3–10 times larger than that of more commonupslope propagating fronts. As the main turbulence is in the interior with a thin stratified layer close tothe bottom, little sediment is resuspended by a downslope propagating front. The downslope propagatingfront is suggested to be generated by oblique propagation of internal (tidal) waves and flow over a nearbyupstream promontory.